This invention is an improvement to the Self-Powered Current Monitor invention disclosed in copending U.S. patent application Ser. No. 09/026,474, filing date Feb. 19, 1998, now U.S. Pat. No. 6,018,700.
This invention relates to electric power monitors, and more particularly to a power monitor that senses power system voltages by capacitive coupling to power system conductors and derives operating power from current information signals.
Power monitors for electric power systems normally require current information signals and voltage information signals that have characteristics that are similar to the power system currents and voltages that are to be monitored. Power quantities (Watts, VARs, Volt-Amperes, Watt-hours, etc.) are normally calculated from the current information signals and voltage information signals.
Most power monitoring systems for a-c (alternating-current) electric power systems utilize current transformers to provide current information signals that are isolated from the electric power system conductors. Voltage information signals are usually obtained by direct connection to power system conductors (with suitable fuses for protection) or by direct connection of a voltage transformer to power system conductors (again, with suitable fuses). Most modern power monitors are electronic and require a small amount of operating power, which is often derived from the voltage information signals. Sometimes a constant-voltage power source (a-c or direct-current) that is separate from the voltage information signals may be used to supply operating power to a power monitor.
When it is desirable to add a power monitor to an existing power system, it is usually necessary to make direct connections to existing power system conductors. These direct connections are often difficult and expensive, and often require a shutdown of the power system. For these reasons, a power monitor that does not require direct connections to power system conductors is desirable.
The Self-Powered Current Monitor invention disclosure (previously referenced) shows power supply circuits that may be used to derive both operating power and information from one or more current information signals, each current information signal being generated by a xe2x80x9ccurrent sourcexe2x80x9d such as a current transformer. The present invention utilizes similar power supply and current sensing circuits as those previously disclosed for the Self-Powered Current Monitor. The present invention is an improvement to the Self-Powered Current Monitor invention, extending the capability of the Self-Powered Current Monitor to include other power system parameters (beyond just current), including voltage and power quantities.
As used herein, the term xe2x80x9ccurrent sourcexe2x80x9d is intended to refer to a source of electricity that generates a specified amount of current with the voltage output varying as required to match various system parameters. The specified current may be constant or time-varying, depending on the application. Standard current transformers (that generate secondary currents proportional to primary currents) act as xe2x80x9ccurrent sources.xe2x80x9d
The term xe2x80x9cvoltage sourcexe2x80x9d means a source of voltage that generates a specified amount of voltage with the current output varying as required to match various system parameters. The specified voltage may be constant or time-varying, depending on the application. Most a-c power systems and d-c (direct current) power supplies are designed as xe2x80x9cvoltage sourcesxe2x80x9d with relatively constant voltage magnitudes and with current levels varying based on load requirements.
It should be noted that some current transformers are configured with an integral resistor and provide an output signal that is a voltage signal proportional to the primary current. This type of current transformer acts like a xe2x80x9cvoltage sourcexe2x80x9d as long as any circuits that are connected have high impedance relative to the internal resistor. Unless specifically noted otherwise, the term xe2x80x9ccurrent transformerxe2x80x9d as used herein refers to current transformers with xe2x80x9ccurrent sourcexe2x80x9d outputs (no integral resistors).
The invention described herein is an electric power monitor that requires no direct connections to power system conductors. Similar to the Self-Powered Current Monitor invention (previously referenced), the present invention can derive operating power from current information signals that are generated by current sources (such as current transformers). The present invention adds voltage and power monitoring capability to the Self-Powered Current Monitor invention.
Adding the capability of voltage and power monitoring (in addition to current monitoring) requires sensing of power system voltages. Power quantities may then be calculated from current and voltage data. To sense voltages without making direct connection to power system conductors, power system voltages are sensed by capacitive coupling. This capacitive coupling may take the form of a conductive material wrapped around the insulation of a power system conductor. This coupling acts like a small capacitor connected to the power system conductor. A second (standard) capacitor, internal to the power monitor, is connected in series with the xe2x80x9ccapacitive-couplingxe2x80x9d capacitor to form a voltage divider. The voltage across the second capacitor is proportional to the power system voltage and is input to an analog-to-digital converter for communication to a microprocessor for analysis.
Some objects and advantages of the present invention are:
(a) Provide a simple way to sense power system voltages without directly connecting to power system conductors.
(b) Provide a power monitor that does not require direct connections to power system conductors.
Further objects and advantages will become apparent from a consideration of the drawings and ensuing description.